Mark R. Rennie

Aero-Optic Effects of a Wing Tip Vortex

For an airborne optical system carried by a helicopter, aero-optic aberrations originate primarily from blade tip vortices that pass through the system field of view; these aberrations result from the reduced pressure and density in the vortex cores and the associated variations in the index-of-refraction. Using the weakly compressible model (WCM) previously developed at Notre Dame, aero-optic effects for realistic tip-vortex flows are computed using the Lamb-Oseen vortex model as well as experimental velocity fields, and compared to measured wavefronts. Scaling relations are developed that enable the prediction of the aero-optic aberrations for full-scale flight vehicles.

Computation of the Aero-Optical Environment of a Helicopter Using Prescribed-Wake Methods

A fast low-order computational approach is presented for estimating the aero-optic effect of the rotor tip vortex system on helicopter-borne optical systems. The approach employs prescribed-wake methods that have been developed by the helicopter design community to rapidly generate realistic approximations to the vortex-wake systems of helicopters in hover and forward flight. With the geometry of the vortex-wake system defined, an experimentally validated computational method that employs the isentropic assumption is then used to compute pressure, temperature, density, and index-of-refraction fields in the selected computational domain. Predictions determined using the computational approach for the spatially and temporally resolved aero-optic aberrations on an optical system mounted on a helicopter in hover and forward flight are presented.

The Aero-Optical Environment of a Helicopter in Forward Flight

The performance of an optical system mounted in a hemispherical turret can be seriously limited at certain viewing angles due to aero-optical aberrations on the outgoing beam. The strength of these aberrations is related to the density fluctuations in the air immediately surrounding the turret and is proportional to the Mach number squared; helicopters are therefore an attractive platform for optical systems due to their low flight speeds. In this case, aero-optic aberrations can still arise, however, from the wake and especially the tip vortices from the helicopter blades. A simplified model of a rotor tip-vortex system is used to gain insight into the potential aero-optical environment of a helicopter in forward flight. The simplified model allows for the rapid calculation of aero-optic effects at different forward-flight speeds. The results indicate that the largest aero-optic aberration originates from “super vortices” that appear along the lateral edge of the helicopter wake. Estimated aberration amplitudes are presented, as well as an estimate of the undisturbed viewing angle of the optical system as a function of forward-flight speed.

The Aero-Optical Environment Around a Helicopter Computed using the Compressible Vorticity Confinement Method

The methodology and results of a computational investigation into the nearfield aerooptic effects on a helicopter-borne optical system are presented. The approach investigated in the study was to incorporate a vorticity-confinement method into a commerciallyavailable CFD code, thereby allowing the rotor wake to be modeled in a less computationally-expensive manner with a higher degree of accuracy. This method is expected to produce more-accurate vortex dynamics, including vortex interactions and other secondary effects, thus leading to improved information regarding the aero-optic environment encountered by an optical system mounted on a helicopter.

Aerodynamic Shaping of Spherical Turrets to Mitigate Aero- Optic Effects

Hemispherical turrets provide an efficient means of mounting an optical system on an aircraft, but are susceptible to the formation of flows, such as shocks and separated shear layers that are known to present a severe optical aberration to the outgoing beam. In this paper, methods are presented to prevent or mitigate the formation of these aero-optic flows using improved aerodynamic shaping of the turret itself. The investigation focuses on the use of the “virtual duct” approach, which has been shown previously to successfully prevent aero-optic flows on an underwing pod up to freestream Mach numbers over 0.8. Computational fluid dynamic results are presented to demonstrate the effectiveness of the virtual duct approach, as well as other mitigation strategies. Nomenclature

Experimental Measurements of an Aircraft-Mounted Pod Concept for Improved Aero-Optic Performance

Experimental data are presented for a wind-tunnel model of an aircraft under-wingmounted pod. The pod model was designed to prevent or mitigate aero-optic aberrations around an optical aperture in a turret ball at the nose of the pod. In particular, the model incorporated a “virtual duct” around the optical aperture, in which fences are attached to the spherical surface of the turret ball and shaped to modify the flow around the turret so as to prevent flows that are known to cause severe optical aberrations. Pressure data are presented that closely compare with inviscid CFD computations, and that indicate that the virtual-duct approach successfully prevents flow separation on the rear of the turret ball. The experimental data also show that the fences of the virtual duct significantly increase the critical Mach number at which supersonic flow and shock formation would occur on the turret ball.

Optimum Design of an Aircraft-Mounted Pod for Improved Aero-Optic Performance

Aero-optic aberrations originating from the nearby flowfield of an aircraft can seriously limit the ability to focus on-board laser systems onto farfield targets. These aero-optic aberrations can be mitigated by using fences to control the flow around the outgoing beam aperture. The objective of this investigation is to attempt to determine the best shape for these fences using computational fluid dynamics in combination with optimization techniques. Future work will experimentally and computationally build on the solutions presented here.

Effect of Acoustic Disturbances on Aero-Optical Measurements

Recent ground-test and flight-test activities have shown that optical wavefront measurements can detect and be significantly influenced by acoustic noise. This finding has implications towards the optical environment of aircraft-mounted optical systems, and also on the effect that background noise could have on optical measurements during wind-tunnel testing. In this paper, results are presented for wavefront measurements through a simple, single-source acoustic field, and compared to theoretical models. Results are also shown for wind-tunnel tests using typicalmodel-support configurations, to show how optical datamight be affected in cases where the acoustic field is a more-complicated superposition of multiple sources plus wall reflections.

Development and Experimental Validation of a Dynamic Model for Wind-Tunnel Heat Exchangers

A discretised dynamic heat-transfer model has been developed for the heat transfer processes occurring in a finned-tube cross-flow heat exchanger. An assumption that the fins and tubes can be treated as a single thermal entity leads to the development of a discretised set of equations that can be solved to generate a predicted heat transfer rate. Comparison of the model with experimental data shows the results obtained are consistent with the transient response of a low-speed closed circuit wind tunnel when subject to a step change in cooling flow to the heat exchanger.